Film Positives and Accurate Image Translation

Dmin is the minimum density of the film itself and refers to its transparency to UV light. Vellums and frosted polyesters used for toner-based systems have a very high Dmin. This means that screen-exposure time must be increased in order to burn through this fog or haze. This, in itself, is not necessarily bad. But Dmin has to be considered in relation to Dmax.

Dmax refers to the maximum density of imaged areas and represents their ability to block UV light in the 365-420 nanometer range. It's important to note that imaged areas only need to fall within this particular range of wavelengths. An image that is somewhat transparent in the visible spectrum can be completely opaque to UV light, which means it would be suitable for screen exposure. For those of you who remember ruby masking films, they're a good example of a visibly transparent but UV-opaque material.

The ideal Dmax density is 4.0 or higher. This value is logarithmic, meaning that a density of 4.0 transmits only 10-4 or 1/10,000 of the light hitting the surface. Lower-end imaging devices will generate Dmax densities in the order of 2.0 -2.5, which may work for large areas of solid art, but are too low for good halftone work. A transmission densitometer is used to determine the Dmin and Dmax values.

The difference between the two Dmax and Dmin values represents the dynamic range of the positive. The minimum dynamic range required for half tones is 2.5 but ideally should be greater than 3.0. To achieve this with most toner positives, the film must be treated with a fusing agent. All of the other imaging system types will give you enough density if they're functioning correctly and have been properly calibrated.

Silver-based positives have the highest dynamic range--I've seen examples with a range in excess of 5.0. But this is overkill. It's better to adjust the laser or thermal heads to provide a dynamic range of 4.0. This will greatly extend imaging head life.

Imagesetter calibration

No imaging device, including a camera, will provide satisfactory halftone positives until it has been linearized. This means adjusting the machine or the RIP (via a transfer curve) to assure that each halftone dot percentage will image at the expected value.

Screen printers do not realize how far off target their positives can be. I have seen brand new imagesetters generate halftones on which the 10% dot averaged 37-41% and the 50% dot imaged around 85-88%. If the user was expecting 10%, he was in for a big surprise. Once the machines were calibrated to produce an ideal transfer curve, none of the tonal values in the 5-99% range were off more than 1%.

It is extremely common for imagesetters to be this far out of calibration, and simply spending money on an expensive imagesetter does not guarantee success. The calibration process itself involves making a test scale or running a premade scale, measuring the results, and adjusting the unit until it provides correct values. For a thorough description of the procedure, refer to the "Calibration" section of the Adobe Photoshop User Manual.

Conclusion

The first step in accurate halftone reproduction is finding the right imagesetting system, one that ensures accurate registration from film to film, delivers ample resolution for your applications, uses stable media, and provides proper optical characteristics for film exposure. Before settling on a particular system, ask the manufacturer to run test samples of images you typically print. This is the only way to be certain that the device is appropriate.

The second step is to calibrate the system. This way, the imagesetter will provide the tonal values you expect for halftone work. The next step is to determine the quality of the stencil on the exposed screen itself, which is a topic for another day.